A duplexer is a radio frequency (RF) component in an RF front end of a radio transceiver used in base stations and wireless devices in a wireless communication system. FIG. 1 shows a well-known duplexer design 2 used in many transceivers. The duplexer 2 has a transmit band pass filter (BPF) 4 and a receive BPF 6. The transmit BPF 4 is connected to a transmitter and is configured to pass signals having frequencies in the transmit frequency band F1, while rejecting signals at other frequencies, including rejecting signals at the receive frequency band F2. The receive BPF 6 is connected to a receiver and is configured to pass signals having frequencies in the receive frequency band F2, while rejecting signals at other frequencies, including rejecting signals at the transmit frequency band F1. Both BPFs 4 and 6 are connected to an antenna. Ideally, transmit signals in the transmit frequency band F1 from the transmitter are passed through the transmit BPF 4 to the antenna but are blocked from reaching the receiver by the receive BPF 6. Likewise, signals received by the antenna in the receive frequency band F2 are passed by the receive BPF 6 but blocked by the transmit BPF 4.
In reality, no BPF provides perfect isolation of out-of-band signal frequencies so that some of the transmit energy from the transmitter will leak through the BPF 6 into the receiver. Further, passive intermodulation (PIM) generated by the transmit BPF 4 may pass the receive BPF 6 and be received by the receiver. Note that since the transmit BPF 4 and the receive BPF 6 are directly connected at the antenna port, a stringent out of band attenuation requirement must be met in order to limit this leakage. Further, the power-handling capability of this type of duplexer design is mainly determined by the transmit BPF design.
Currently, only two types of small duplexers are commercially available: an acoustic type and a ceramic type. The acoustic type may be a surface acoustic wave (SAW), bulk acoustic wave (BAW) or film bulk acoustic resonator (FBAR). The ceramic type includes monoblock duplexers and ceramic coaxial duplexers. Whether the acoustic type or the ceramic type is used may depend on the power handling requirements of the transceiver and the maximum leakage tolerable at the receiver. The choice further depends on size, cost and weight constraints.
In general, for a radio design with transmit power averaging less than about 23 dBm, the acoustic type duplexers can meet entire performance requirements of the handset designs, but cannot fully meet the performance requirements of some base stations such as small cell base station designs. Some base stations require very high isolation between the transmit and receive ports, especially in the cases of high transmit power, which have not been achievable by acoustic type duplexer designs, and consequently, ceramic filters are typically used in these cases.
A disadvantage of ceramic filters is their size. A typical ceramic type duplexer may be of the dimensions of 52×14×6 millimeters (mm), whereas a typical acoustic type duplexer may be of the dimensions 2×1.6×0.6 mm. Hence, a typical ceramic type duplexer may be over 2000 times larger than an acoustic type duplexer. In addition to large size, ceramic type duplexers may be 100 times heavier and 10 times more costly than acoustic type duplexers. Further, acoustic type duplexers have Q factor that may be three times greater than the Q factor of ceramic type duplexers. A Q factor is an indication of energy stored by a resonator divided by energy dissipated per cycle.
Advantage to the ceramic type duplexers over other designs include much higher transmit power handling capability and lower PIM at the receive port for the same power handling. Therefore, designs for high power with low PIM requirements may be limited to ceramic duplexers.
FIG. 2 shows a duplexer design 8 that can be used for higher power applications. The duplexer 8 has two electrically parallel transmit BPFs 4a and 4b, referred to collectively as BPFs 4, a receive BPF 6, and two 90° hybrid couplers 10a and 10b referred to collectively herein as hybrid couplers 10. A 90° hybrid coupler is a four port device that is used either to equally split an input signal into two paths or to combine two signals while maintaining isolation between them.
For example, the 90° hybrid coupler 10a splits the input from the transmitter at port A into two equal magnitude signals that are output at ports B and C. In this example, port D is terminated with a 50 ohms load. Each output of the 90° hybrid coupler 10a is input to a different transmit BPF 4. Each BPF 4 has a substantially identical band pass response configured to pass signals at a transmit frequency band F1. Each transmit BPF 4 is output to one of the inputs of the 90° hybrid coupler 10b via ports B and C.
The 90° hybrid coupler 10b combines the inputs at ports B and C and outputs the combined signal at port A to an antenna. A signal received from the antenna is received at port A and split to two paths towards ports B and C, respectively. The split two signals are reflected at the ports B and C, and the reflected signals are combined at port D, which is coupled via the receive BPF 6 to a receiver. Since two band pass filters are used to filter the transmit signal, this type of duplexer might handle twice the transmit power of a duplexer having only one transmit BPF. Also, due to signal cancellation provided by the 90° hybrid couplers 10, this type of duplexer has much lower PIM at its receive port and much higher isolation between the transmit and receive ports of the duplexer as compared to the duplexer of FIG. 1.
However, if the two BPFs 4 were designed in SAW, BAW or FBAR filter technology, the balanced duplexer design of FIG. 2 can handle only twice the power of the duplexer design of FIG. 1 which is inadequate to meet the demands of a wide range of high power applications. Thus, in many cases, the bulky, heavy and expensive ceramic type duplexers are still used.